A dihydropyrane compound is an important industrial raw material for perfumes. For example, .alpha.-phenyl-dihydropyrane can be converted to 5-phenylpentanol which is particularly important as perfume by reductive ring opening of the pyrane ring (Swiss Patent 655932). Further, dihydropyrane compounds such as 6-phenyl -4-methyl-5,6-dihydro-2H-pyrane, 6-phenyl-2,4-dimethyl-5,6-dihydro-2H-pyrane and 6-butyl-2,4-dimethyl-5,6-dihydro-2H-pyrane are useful themselves (U.S. Pat. No. 3,681,263 and Arm. Khm. Zh. (1976), 29 (3), p. 276 to 277).
As described in the literatures described above, these dihydropyrane compounds can be obtained in the form of a mixture of the double bond isomers by reacting aldehyde compounds such as benzaldehyde and valeraldehyde (pentanal) with 3-butene-1-ol compounds such as isoprenol in the presence of a catalytic amount of an acid. However, 3-butene-1-ol compounds are expensive, and therefore a method for preparing these dihydropyrane compounds from readily available and inexpensive raw materials has been desired.
A method by a hetero Diels-Alder reaction of aldehyde compounds with conjugated diene compounds is known as such a method. In this case, conjugated diene compounds such as isoprene and 2-methylpentadiene are readily available. In general, however, in this kind of the hetero Diels-Alder reaction, the products have been able to obtain at practical reaction yield only when high reactive aldehyde compounds such as glyoxylic ester and trichloroacetaldehyde are used (Comprehensive Organic Synthesis, Vol. 5, p. 431 Pergamon Press 1991).
A reaction of aldehyde compounds with diene compounds using a Lewis acid as a catalyst is available as an improving method thereof. For example, a method in which aluminum chloride or tin tetrachloride is used as a Lewis acid catalyst and an aliphatic or aromatic nitro compound is further used as a co-catalyst is known (JP-A 1-238578). In this method, however, wastes are produced in large quantities after the reaction, and the reaction yield is as low as about 50%. Accordingly, this method has not yet been sufficiently satisfactory in the viewpoint of reaction yield and productivity. In recent year, known as well is a method in which rare earth metal perfluoroalkanesulfonate such as scandium perfluorate as a catalyst is used to carry out the hetero Diels-Alder reaction (New Journal of Chemistry, 1995, vol. 19, 707). However, this method uses an expensive catalyst and therefore is uneconomical and industrially unsuitable.
Tetrahedron Letters vol. 38, No. 14, pages 2569-2572, published on, Apr. 7th, 1997, discloses reaction of an aromatic aldehyde with an excess amount of a diene with a catalsyt of trifluoromethanesulfonic acid.
Accordingly, an object of the present invention is to provide a simple process for preparing a dihydropyrane compound by the hetero Diels-Alder reaction of an aldehyde compound with a diene compound, in view of a high productivity, a high reaction yield and an economic save.